There are various known methods for communicating between communications apparatuses, and various multiplexing communication methods are proposed in response to demands for achieving large capacity transmission.
For example, TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), CSMA (Carrier Sense Multiple Access), and CDMA (Code Division Multiple Access) are known and are also used in wireless communications.
Further, various systems, in combination with the multiplexing communication methods, are known for achieving wireless communications between many mobile stations or between a base station and plural mobile stations.
A mobile phone system may have plural base stations allocated in a scattered manner so that service areas partly overlap each other. Thereby, communications between a mobile phone and other mobile phones or stationary telephones can be established while the mobile phone is being carried on the move, and the base stations can assign channels to each mobile phone.
Further, there is proposed a control method in which a mobile station requests a base station to assign a channel from a shared channel by using a CSMA/CA method, and the base station performs scheduling of transmission time by using TDMA (see, for example, Patent Document 1).
Further, there is proposed a system and a method that periodically divide a beacon region (where beacon packets are transmitted from plural master stations (stationary stations)), a TDMA region (where specific authorized stations use bandwidths assigned thereto), and a CSMA region (where competitive access is made) on a time-line and allow selection of a region enabling bandwidth guarantee (see, for example, Patent Document 2).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-374265    Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-73240
In a case where plural mobile stations perform wireless communications directly with other mobile stations, communication channels in a predetermined frequency bandwidth are to be assigned in a manner that the mobile stations do not interfere with each other.
In a case where a base station (stationary station) has almost all areas as its service areas, the base station (stationary station) can assign communication channels to mobile stations and control the communication channels. However, in a case where plural base stations are scattered at strategic locations along a path or the like or where the base station has an event site as its service area, a mobile station cannot receive the service of being assigned a communication channel unless the mobile station moves to a service area (hot spot) of the base station(s).
Accordingly, wireless communications between mobile stations are performed by using, for example, a slotted aloha method (in which frames and slots are synchronized between the mobile stations) and non-colliding communication channels are autonomously selected. Further, the slotted aloha method, for example, is used for establishing multiple access among mobile stations whereas the TDD (Time Division Duplexing) method, for example, is used for establishing wireless communications between mobile stations and base stations.
With this mobile wireless system, a mobile station determines the timing for performing transmission according to the TDD slot timing of a base station when the mobile station approaches the base station and communicates in synchronization with the TDD slot timing of the base station when the mobile station communicates with the base station.
However, in a case where no base station exists in the vicinity of a mobile station, the mobile station, unlike using a cellular wireless method in which the mobile station can obtain a slot timing signal from a base station, is required to continue to autonomously obtain the timing for synchronizing with the base station.
Therefore, there is conceived a synchronizing method in which accurate time data are obtained from a GPS (Global Positioning System) for synchronizing with the slot timing. In order to continue obtaining time data from radio-waves received from a GPS satellite, it is necessary to continue receiving radio-waves constantly from four or more GPS satellites in order to receive the time data.
In a case where the mobile station is located in an area where the status of receiving radio-waves from the GPS satellites is unsatisfactory, it is difficult for the mobile station to obtain time data from the radio-waves of the GPS satellites. Therefore, in some cases, it may be difficult to autonomously continue synchronization.
Further, in a case where no base station exists in the vicinity of a mobile station and the status of received radio-waves from GPS satellites is unsatisfactory, it is possible to perform wireless communications by using, for example, the slotted aloha method.
However, in order to efficiently perform wireless communications by making the length of time of a time slot (time slot length) used by the slotted aloha method substantially equal to the length of time of a time slot (time slot length) designated by TDD slot timing used by the base station according to the TDD method, it is necessary to receive radio-waves from GPS satellites and continue maintaining slot synchronization by obtaining accurate time data from the radio-waves.
In this case, it is often difficult for the mobile station to constantly obtain accurate time data. Accordingly, continuing to maintain synchronization of high accuracy is difficult, and wireless communications of other mobile stations may be obstructed by such difficulty.